Density determination of solid plastics is a fundamental physical test used to identify polymers, verify material quality, and evaluate crystallinity or filler content. This article provides a comprehensive technical overview of solid plastic density testing, covering applicable standards such as ISO 1183 and ASTM D792, sample requirements, testing methods, and typical applications in quality control and materials research.
Technical Overview
Density is one of the most important physical properties of plastic materials, defined as the mass per unit volume of a material at a specified temperature, typically expressed in grams per cubic centimeter (g/cm³) or kilograms per cubic meter (kg/m³). For solid plastics, density measurement serves as a rapid and reliable indicator of material identity, homogeneity, degree of crystallinity, and filler or reinforcement content.
Because polymers are sensitive to temperature, density values are conventionally reported at a standard laboratory temperature of 23 °C unless otherwise specified. Two related quantities are frequently reported in plastics testing: bulk density, which accounts for voids between particles in powders or pellets, and solid density, which describes the intrinsic density of the material itself. Another commonly used term is specific gravity (relative density), the ratio of the density of a material to the density of water at a reference temperature. Since the density of water is approximately 1.0 g/cm³, specific gravity and density values are numerically close for plastics.
Density differences among polymers arise from variations in molecular structure, chain packing, crystallinity, and the presence of additives, fillers, or reinforcements. For example, polypropylene and polyethylene have densities below 1.0 g/cm³, causing them to float in water, whereas polycarbonate, polyamide, and glass-filled compounds have densities above 1.0 g/cm³. This characteristic makes density testing a powerful first-line tool for material identification and incoming inspection of plastic resins and finished products in accredited third-party laboratories.
Testing Samples
Molded Specimens and Finished Parts
Injection-molded plaques, compression-molded sheets, and representative sections cut from finished products can be tested directly, provided the material is free of voids, bubbles, and surface contamination. Finished parts should be cut into suitable pieces so that surface tension effects do not distort the measurement.
Pellets and Granules
Virgin resin pellets, granules, and regrind materials are commonly tested for incoming quality control. Individual pellets or small groups of pellets may be measured using the immersion method or the density gradient column technique.
Films and Sheets
Thin films can be tested by stacking multiple layers or cutting larger pieces to achieve adequate sample mass. Care must be taken to exclude entrapped air between layers.
Powders and Small Particles
Fine powders, such as polymer micropowders or composite fillers, are typically characterized using a gas pycnometer, which measures true skeletal density without interference from interparticle voids.
Sample Requirements
- Sample mass: typically at least 1 g for the immersion method; smaller amounts may be tested with a density gradient column or microbalance setup.
- Sample condition: clean, dry, free of lubricants, mold release agents, and surface coatings that could alter the measured mass or volume.
- Conditioning: samples should be conditioned at 23 ± 2 °C and 50 ± 10% relative humidity prior to testing, particularly for hygroscopic polymers such as polyamide.
- Specimen integrity: samples must be free of closed voids, as internal porosity leads to falsely low density readings.
Testing Items
Solid plastic density testing typically covers the following measurement parameters:
- Solid density (g/cm³): the intrinsic density of the plastic material measured at 23 °C.
- Specific gravity (relative density): dimensionless ratio of material density to water density, reported with reference temperatures (e.g., 23/23 °C).
- Apparent density: mass per unit volume including internal voids, relevant for foamed or porous materials.
- Bulk density: applicable to pellets and powders, reflecting packing behavior rather than intrinsic material density.
- Density of filled and reinforced compounds: used to estimate filler or glass fiber content through comparison with the neat polymer baseline.
Testing Methods
Immersion Method (Hydrostatic Method)
The immersion method, described in ISO 1183-1 (Method A) and ASTM D792 (Method A), is the most widely used technique. The specimen is weighed in air using an analytical balance, then re-weighed while fully immersed in a wetting liquid, usually distilled water with a trace of wetting agent. Based on the Archimedes principle, the buoyancy force equals the weight of the displaced liquid, allowing volume to be calculated from the mass difference. Density is then obtained by dividing the mass in air by the calculated volume.
This method offers excellent accuracy (typically within 0.001 g/cm³), short measurement time, and suitability for samples with regular or irregular geometry. For materials with density lower than 1.0 g/cm³, a sinker or immersion aid is attached to submerge the floating specimen, and corrections are applied accordingly.
Liquid Pyknometer Method
Specified as ISO 1183-1 (Method B) and ASTM D792 (Method B), the liquid pyknometer method measures the mass of displacement liquid displaced by the sample inside a calibrated flask. It is particularly suited to small samples such as pellets, powders that do not dissolve, and cut fragments. The method requires careful temperature control and degassing of the liquid to avoid air bubble errors.
Titration Method
The titration method (ISO 1183-1, Method C) determines density by observing whether small specimens float, sink, or remain suspended in a binary liquid mixture whose density is adjusted by titration. When the specimen reaches neutral buoyancy, the density of the liquid equals that of the sample, and the liquid density is measured with a precision hydrometer or oscillating U-tube densitometer. This approach is useful for very small samples and for quality control screening.
Density Gradient Column Method
Described in ISO 1183-2 and ASTM D1505, this technique uses a vertical column of liquid whose density increases in a calibrated, linear gradient from top to bottom. Calibrated glass floats of known density are placed in the column, and the test specimen is dropped in; its equilibrium position indicates its density through interpolation. The method achieves high precision (approximately 0.0001 g/cm³) and is ideal for distinguishing polyethylene grades with very close densities, such as differentiating medium-density from high-density polyethylene.
Gas Pycnometer Method
Specified in ISO 1183-3, gas pycnometry measures volume by gas displacement, typically using helium, which penetrates fine surface texture and open pores. This method yields true skeletal density and is preferred for powders, porous materials, and samples that would be damaged or altered by contact with liquids.
Typical Density Values of Common Plastics
- Polypropylene (PP): 0.890–0.910 g/cm³
- Low-density polyethylene (LDPE): 0.910–0.925 g/cm³
- High-density polyethylene (HDPE): 0.941–0.965 g/cm³
- Polystyrene (PS): 1.04–1.06 g/cm³
- Acrylonitrile butadiene styrene (ABS): 1.03–1.08 g/cm³
- Polymethyl methacrylate (PMMA): 1.17–1.20 g/cm³
- Polycarbonate (PC): 1.20–1.22 g/cm³
- Polyamide 6 (PA6): 1.12–1.15 g/cm³
- Polyethylene terephthalate (PET): 1.33–1.40 g/cm³
- Polyoxymethylene (POM): 1.40–1.42 g/cm³
- Rigid polyvinyl chloride (PVC): 1.35–1.50 g/cm³
- Polytetrafluoroethylene (PTFE): 2.12–2.20 g/cm³
- Glass-fiber reinforced compounds: typically 1.20–1.60 g/cm³ depending on fiber loading
Key Standards Reference
- ISO 1183-1: Plastics — Methods for determining the density of non-cellular plastics — Part 1: Immersion method, liquid pyknometer method and titration method
- ISO 1183-2: Density gradient column method
- ISO 1183-3: Gas pyknometer method
- ASTM D792: Standard Test Methods for Density and Specific Gravity (Relative Density) of Plastics by Displacement
- ASTM D1505: Standard Test Method for Density of Plastics by the Density-Gradient Technique
- GB/T 1033: Chinese national standard for determining the density of non-cellular plastics, technically aligned with ISO 1183
Applications
Material Identification and Verification
Density measurement provides a rapid screening tool to confirm that received resins or molded parts match the specified polymer. It is often combined with spectroscopic techniques such as infrared analysis for definitive identification.
Incoming Inspection and Quality Control
Resin producers, compounders, and plastic processors use density testing to verify lot-to-lot consistency and to confirm compliance with material datasheets and purchase specifications before production.
Polyethylene Grade Classification
Because polyethylene grades are formally classified by density, the density gradient column and immersion methods are essential for distinguishing LDPE, LLDPE, MDPE, and HDPE in both production certification and market surveillance.
Crystallinity Evaluation
The density of semi-crystalline polymers increases with crystallinity because crystalline regions pack more tightly than amorphous regions. Density measurements can therefore be used to estimate the degree of crystallinity and to monitor the effects of processing conditions such as cooling rate and annealing.
Filler and Reinforcement Content Estimation
The density of a filled compound depends on the volume fraction and density of the filler, such as calcium carbonate, talc, talc, glass fiber, or barium sulfate. By comparing the measured density with the calculated theoretical density, filler content can be estimated for formulation control.
Failure Analysis and Defect Investigation
Abnormally low measured density can indicate internal voids, insufficient packing during molding, or degradation, making density testing valuable in root cause analysis of failed components.
Recycling and Waste Sorting
Density-based sink-float separation is a cornerstone of plastic recycling operations, enabling sorting of polyolefins from heavier polymers such as PVC and PET in material recovery facilities.
FAQ
What is the difference between density and specific gravity?
Density is an absolute property expressed in mass per unit volume (g/cm³), while specific gravity is a dimensionless ratio of the material density to the density of water at a specified reference temperature. Because water has a density close to 1.0 g/cm³ at room temperature, the two values are numerically similar for most plastics, but the reference temperatures should always be stated with specific gravity results.
Which method should be selected for my sample?
For molded parts and samples with sufficient mass, the immersion method (ISO 1183-1 Method A or ASTM D792 Method A) is generally preferred for its speed and accuracy. For pellets and small fragments, the pyknometer or density gradient column methods are suitable. For powders and porous materials, gas pycnometry is recommended.
How much sample is required?
A minimum mass of approximately 1 g is recommended for the immersion method to ensure adequate measurement precision. The density gradient column can accommodate samples of only a few milligrams, while gas pycnometry requires enough volume to displace a measurable quantity of gas, typically several cubic centimeters.
Why is temperature control important?
Both plastic density and the density of the immersion liquid vary with temperature. A temperature deviation of just 1 °C can introduce errors on the order of 0.0005 g/cm³ or more. All measurements are therefore performed at 23 ± 0.5 °C with temperature-corrected liquid density values.
Can hygroscopic plastics be tested accurately?
Yes. Materials such as polyamide absorb moisture, which alters both mass and volume. Samples should be dried to a consistent condition or conditioned according to the relevant standard before testing, and the conditioning state should be reported with the results.
How does moisture or trapped air affect results?
Surface bubbles clinging to the specimen cause an apparent density that is too low, which is why a wetting agent is added to the immersion liquid and samples are gently brushed or agitated to release adhered air. Closed internal voids likewise reduce measured density, so specimens must be void-free for accurate solid density determination.
What accuracy can be expected?
The immersion method typically achieves repeatability within about 0.001 g/cm³, the liquid pyknometer method within about 0.0005 g/cm³, and the density gradient column method within about 0.0001 g/cm³ when performed by experienced personnel in accredited third-party laboratories.